Petrogenesis of granitoids in the vicinity of the Mactung Tungsten Skarn Deposit, NE Yukon-Northwest Territories: characterization of skarn mineralization and causative plutons through geological, petrochemical, mineralogical and geochronological analysis

Abstract

Skarn W-(Au-Cu-Bi) deposit at Mactung, Yukon, Canada is situated within the eastern flank of the NW-SE striking poly-deformed Paleozoic Selwyn metasedimentary basin. Two biotite granite stocks occur south and north of the deposit. The northern stock is dominantly porphyritic, cut and rimmed by a more leucocratic medium- to coarse-grained granite. Prominent aplitic dykes outcrop south of the mineralization. Major- and trace-elements strongly discriminate between the various phases of biotite granite and the leucogranite. The biotite granite has an arc-like affinity. The leucogranite is sourced from anatexis of a supracrustal sequence at depth. Tracer Sm-Nd and Rb-Sr isotope systems further indicate the granitoids were sourced from the partial melting of an old continental crust. The δ[superscript 18]O values of Mactung granitoids indicate strong metasedimentary contamination from the source region or during magma ascent. The less evolved biotite granite is directly associated with the W-(Au-Cu-Bi) mineralization, based on field relationships, metallogenic aspects of the intrusion, alteration-mineralization effects, and previously published Re-Os dating of molybdenite and new geochronological data. Multiple dating techniques were used to date various rock types of Mactung, northeast Yukon. Ar-Ar dating reveals the following best ages: 95.6 ± 0.3 to 98.1 ± 2.0 Ma from muscovite grains, and 91.8 ± 0.4 to 95.1 ± 0.8 Ma from biotite grains of porphyritic biotite granite; 93.3 ± 1.2 Ma from biotite grains of coarse-grained leucogranite; 92.9 ± 0.4 and 95.3 ± 0.4 Ma from muscovite grains of medium- to coarse-grained leucocratic granite dyke; 94.0 ± 0.5 Ma from biotite grains of an aplitic dyke; 97.1 ± 1.9 and 96.9 ± 0.6 Ma from biotite grains of biotite hornfels. U-Pb age data for zircon grains were obtained by ID-TIMS analytical techniques for five samples from three rock types of Mactung granitoids: an aplitic dyke south of the Mactung skarn tungsten deposit yields igneous crystallization age of 97.1 ± 0.2 Ma; porphyritic biotite granite from the main phase of the Mactung pluton yielded a crystallization age of 97.6 ± 0.2 and 97 ± 0.1 Ma. A leucocratic granite dyke, a marginal phase in the southeast of the pluton, gives an age of 97.0 ± 0.3 Ma. Combining the Ar-Ar and U-Pb age data, it is proposed that injection of leucogranite magma from 97-92 Ma to have prolonged the thermal regime of the area resulting in overprinting and partial resetting (beyond their blocking temperature) of the Ar-Ar ages. A titanite age from the lower skarn orebody was obtained using laser ablation inductively coupled plasma-mass spectrometry that yielded a less precise [superscript 206]Pb/[superscript 238]U concordia intercept age of 97.1 ± 4.1 Ma that agrees very well with the U-Pb zircon dates (this work) of nearby granite intrusive rocks. Re-Os molybdenite dating of quartz veins, cutting all granitoids, and the lower metasedimentary unit, adjacent the Mactung tungsten skarn yields ages of 97.3 to 106.3 ± 0.4 Ma. Variation of Re-Os age dates, relative to the uniform U-Pb age dates, is likely due to [superscript 187]Os and or [superscript 187]Re heterogeneity in the molybdenite grains. The vein age provides a direct lower timing constraint for granite intrusion and sulphide mineralization events of the area, if the older ages are disqualified. The chemical composition of primary and accessory minerals of Mactung granitoids was determined using Electron Probe Microanalysis (EPMA) and SEM Energy Dispersive Spectroscopy (SEM-EDS). The minerals analysed included biotite, feldspars, muscovite, chlorite, apatite, tourmaline, and monazite. Biotite grains from skarn and hornfels (Unit 3C) were also investigated. Differences in the chemical composition of biotite and feldspar are the most robust means of distinctively characterizing rock types. Aluminium Fe, and Mg are the main distinguishing elements between biotite grains of different lithologies. The Mg, Fe, and Al compositions of biotite from Mactung granitoids and the high Fe and Al contents of tourmaline suggest a peraluminous magma source, derived from the melting of the continental crust, possibly in a collisional tectonic setting. A strongly contaminated and reduced I-type magma is implicated from the log X[subscript F]/X[subscript OH] and log X[subscript Mg]/X[subscript Fe] composition of biotite. Non-mineralizing granites of Mactung (leucogranite and aplitic dykes) have high X[subscript Fe], Al[superscript iv], lower X[subscript Mg], and fO[subscript 2] values relative to the biotite granite and ore zone granites, which are believed to be sources of the skarn tungsten mineralization at Mactung. These latter granitoids are characterized by lower IV(F/Cl) intercept ratios relative to porphyry copper-bearing intrusive systems, and higher IV(F) values relative to porphyry Mo and Sn-W-Be bearing systems. Tungsten mineralization at Mactung is contained in scheelite, associated with or without pyrrhotite, chalcopyrite, and pyrite, in pyrrhotite-pyroxene skarn, (garnet) - pyroxene skarn, and pyroxene-pyrrhotite skarn, with anorthitic plagioclase and quartz. In pyrrhotite skarn, pyrrhotite and chalcopyrite are intimately associated, and their abundance is often indicative of higher grade copper. High Au is directly correlated to Bi. Iron, Ag, Sb, Co, Pb, and Cu are also positively correlated to Au. From SEM and EPMA, Au is associated with native bismuth, Te-bearing bismuth phases as solid solution; native bismuth is the dominant phase. Sphalerite and Aluminum-in-hornblende geobarometry show that skarn formed at 2 kb (200 MPa), which corresponds to a depth of 7-8 km. Mineral assemblages of contact metamorphism in hornfelsic rocks indicate temperatures of 585–635°C for a pressure of 2 kb (200 MPa). The host limestone beds were subjected to about 560–675°C, based on solvus thermometric data obtained from coexisting dolomite and calcite. The temperature of formation of coexisting pyrrhotite and pyrite is about 640oC, which corroborates well with the calcite-dolomite solvus thermometer and contact mineral assemblage in hornfelsic rocks. The oxygen isotope fractionation factor between quartz and scheelite, for samples taken from quartz veins, indicate a corresponding temperature value of 558°C. At this temperature, the early stage pyroxene and scheelite probably crystallized at an oxygen fugacity of 10[superscript -17.5] bars or less. Sulphur isotope analysis of sulphides in the skarn system ranges from 9-19‰ relative to VCDT with an average of 13‰. A mixed source of fluids (magmatic and local sedimentary) is hypothesized for this skarn system.